How to implement a Singleton design pattern in C#?

Singleton Pattern belongs to Creational type pattern

Singleton design pattern is used when we need to ensure that only one object of a particular class is Instantiated. That single instance created is responsible to coordinate actions across the application.

As part of the Implementation guidelines we need to ensure that only one instance of the class exists by declaring all constructors of the class to be private. Also, to control the singleton access we need to provide a static property that returns a single instance of the object.

Example

Sealed ensures the class being inherited and object instantiation is restricted in the derived class

Private property initialized with null

ensures that only one instance of the object is created

based on the null condition

Private constructor ensures that object is not instantiated other than with in the class itself

Public method which can be invoked through the singleton instance

 Live Demo

public sealed class Singleton {
   private static int counter = 0;
   private static Singleton instance = null;
   public static Singleton GetInstance {
      get {
         if (instance == null)
         instance = new Singleton();
         return instance;
      }
   }
   private Singleton() {
      counter++;
      Console.WriteLine("Counter Value " + counter.ToString());
   }
   public void PrintDetails(string message) {
      Console.WriteLine(message);
   }
}
class Program {
   static void Main() {
      Singleton fromFacebook = Singleton.GetInstance;
      fromFacebook.PrintDetails("From Facebook");
      Singleton fromTwitter = Singleton.GetInstance;
      fromTwitter.PrintDetails("From Twitter");
      Console.ReadLine();
   }
}

Output

Counter Value 1
From Facebook
From Twitter